Among conventional refrigeration systems provided with a vapor compression type refrigerant circuit, there is an air conditioner design configured such that a portion of the refrigerant flowing through a main refrigerant circuit can be made to bypass the remainder of the main refrigerant circuit so as to return to the intake side of a compressor and used to cool the refrigerant flowing through the main refrigerant circuit to a subcooled state. An air conditioner configured in this fashion is provided with the following: a main refrigerant circuit including a compressor, a heat-source-side heat exchanger and a user-side heat exchanger; a bypass refrigerant circuit connected to the main refrigerant circuit in such a manner that a portion of the refrigerant flowing from the heat-source-side heat exchanger to the user-side heat exchanger branches away from the main refrigerant circuit and returns to the intake side of the compressor; a bypass expansion mechanism that is provided in the bypass refrigerant circuit and configured to regulate the flow rate of the refrigerant flowing through the bypass refrigerant circuit; a cooling device configured to cool the refrigerant flowing from the heat-source-side heat exchanger to the user-side heat exchanger in the main refrigerant circuit using the refrigerant that is returned from the outlet of the bypass expansion mechanism to the intake side of the compressor; a superheating degree detecting mechanism that is provided in the bypass refrigerant circuit and configured to detect the degree of superheating of the refrigerant at the outlet side of the cooling device; and an expansion mechanism control means configured to control the bypass expansion mechanism based on the superheating degree detected by the super heating degree detecting mechanism such that the superheating degree of the refrigerant flowing through the bypass refrigerant circuit is equal to or higher than a prescribed superheating degree.
When an air conditioner configured in this fashion is operated in cooling mode, a portion of the liquid refrigerant that is sent from the heat-source-side heat exchanger to the user-side heat exchanger in the main refrigerant circuit is diverted from the main refrigerant circuit and returned to the intake side of the compressor through the bypass refrigerant circuit (which branches from the main refrigerant circuit) while the flow rate of the diverted refrigerant is adjusted by the bypass expansion mechanism. The refrigerant that flows from the outlet of the bypass expansion mechanism in the bypass refrigerant circuit toward the intake side of the compressor passes through the cooling device and exchanges heat with the liquid refrigerant flowing from the heat-source side heat exchanger to the user-side heat exchanger. After passing through the bypass expansion mechanism, the temperature of refrigerant in the bypass refrigerant circuit is lower than the temperature of the refrigerant flowing from the heat-source-side heat exchanger to the user-side heat exchanger in the main refrigerant circuit. Consequently, the refrigerant in the bypass refrigerant circuit cools the liquid refrigerant flowing from the heat-source-side heat exchanger to the user-side heat exchanger in the main refrigerant circuit and, in turn, is heated. Since the bypass expansion mechanism is controlled by the expansion mechanism control means such that the superheating degree of the refrigerant at the outlet of the cooling device in the bypass refrigerant circuit, i.e., the superheating degree detected by the superheating degree detecting mechanism, is equal to or higher than a prescribed superheating degree, the refrigerant flowing through the bypass refrigerant circuit passes through the cooling device and is heated to the prescribed superheating degree or above before returning to the intake side of the compressor. Meanwhile the refrigerant flowing through the main refrigerant circuit of the cooling device is cooled to a subcooled state corresponding to the amount of heat exchanged with the refrigerant flowing through the bypass refrigerant circuit of the cooling device. In this way, the air conditioner executes superheating degree control in such a manner that the refrigerant flowing through the main refrigerant circuit is cooled to a subcooled state. (See, for example, Japanese Laid-Open Patent Publication No. 07-4756.)